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Ductile iron pipes - - A common defect of nodularizer improper application

by:XEX     2020-12-19
Ductile iron pipes - - Nodularizer improper application of the common defects of casting' target='_blank'>ductile pipe casting defects such as inclusions, holes, cracks, Refers to the pores, lock hole, crack, cold insulation, etc. ) Often affects the mechanical properties of castings, physical and chemical properties, processability, decide the quality of the castings. Ductile iron pieces can produce almost all the casting defects, but because of its production mode, the crystallization law, casting performance and other casting alloy, lead to ductile iron is often appear some special defects. So related to the nodularizer ball iron deficiency, or caused by the factor of nodularizer ball iron deficiency have? In this paper, research has shown that almost all the ball iron defects associated with spheroidizing agent. This basically has the following several aspects: ( 1) Graphite ball: alienation appears irregular graphite, graphite ball alienation, such as crumb, tadpole shape, worm-like, angular, or other balls. This is due to the growth of spheroidal graphite along the direction of radiation, the local and the growth rate of crystal growth pattern caused by deviation from normal growth rule. Castings in the residual amount of spheroidizing element is beyond the scope should be, such as residual magnesium is too high, more than the minimum required to keep the graphite spheroidizing, may also affect the graphite crystallization conditions, strong graphite is prone to tadpoles. Residual rare earth is large, and the high carbon equivalent molten iron is easy to produce pieces of graphite, fragments of graphite area commonly known as a 'resistant'. And the emergence of vermicular graphite is caused by a lack of spheroidizing element residual amount or contains overrun of titanium and aluminum. ( 2) Graphite floating: eutectic composition of thick squash iron piece, on the top of the pouring position, often appear a dense regions of graphite, the beginning and end of the 'floating' phenomenon, this is because the graphite with different hot metal density, hypereutectic molten iron caused by direct separation of graphite by buoyancy effect up. Degree of graphite floating and spheroidizing elements carbon equivalent, and the residue, casting solidification time, pouring temperature and other factors. Magnesium can increase the eutectic carbon content of ductile iron, carbon equivalent of the same hot metal, increase the amount of residual magnesium can reduce graphite floatation, the residual amount of rare earths, help to burst up into graphite. ( 3) Inverse chill: general cast iron pieces in white organization easy to appear in the surface layer of rapid cooling, sharp corners, the place such as cover with seam, inverse chill defect, on the other hand, carbide phase in casting, hot section of medium section the core and other parts. Spheroidizing element residual quantity too much, increase the role of the inverse chill defect, rare earth elements than magnesium, they can generally increase ductile iron formed the supercooling degree of organization. ( 4) Hypodermic needle: within the hypodermic needle mainly contains hydrogen, there are also a small amount of carbon monoxide and nitrogen. When the amount of residual magnesium, also at the same time to strengthen the tendency to absorb hydrogen from the wet, increase chance of generating a hypodermic needle. In addition, the spheroidizing molten iron long residence time can also increase the number of pinhole. ( 5) Porosity shrinkage, shrinkage cavity often appear in the final solidification casting parts ( Hot place, riser neck and casting joint, an internal Angle or gate and casting joint) , is hidden within the casting or holes connected with looks. Shrinkage, macro in hot section, small shrinkage holes, mostly inside the holes connected each other. Is related to the spheroidizing elements, to control the residual magnesium and rare earth can not too high, to reduce the macroscopic and microscopic shrinkage has obvious effect, shrinkage tendency almost is proportional to the spheroidizing elements. ( 6) Black slag: it usually occur in the upper portion of the casting ( Pouring position) , mainly divided into blocks, rope shape and finely residue. The main components of the residue, magnesium silicate, is made up of MgO style in the molten iron and SiO2 reaction generated, and is influenced by its relative content. Therefore, as one of the measures to control the residue is to reduce the amount of residual magnesium ( Adding magnesium 0. Total accounted for about 15%, the slag ladle weight of 0. 1%) Residual rare earth with oxygen, and has a strong affinity, has the obvious effect in reducing residue. ( 7) Spheroidizing recession: this is because the spheroidizing stay hot for a long time, gradually reduce the residual magnesium, grilled slag not timely, will get back in the molten iron, sulfur solidifying graphite reduced or even disappear in the organization, and decay is irregular shape, vermicular or flake graphite. The spheroidizing recession and low rare earth content in nodularizer, or low for the amount of nodularizer has a certain relationship, but close to increase its addition amount is not desirable, because of the high residual amount of magnesium, slag quantity and cementite will increase, in the heavy section will cause the graphite graphite ball into tadpoles. Production practice shows that the original low sulphur content in molten iron is the most effective to prevent the spheroidizing recession. Including the ball iron casting defects and, with almost all the amount of the composition of nodularizer and have a relationship, but we can't expect nodularizer to solve many problems, more can't solve all problems, because the role of spheroidizing elements and spheroidizing agent are the advantages and disadvantages coexist, the dosage of nodulizing agent is the production of nodular cast iron stability control system is a very important factor, and other supporting measures only together, to the stability of the spheroidizing treatment.
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